1. Technical Field
The present invention generally relates to fuel cells; and more particularly, to improvements in performance of polymer fuel cells.
2. Background of the Invention
Polymer fuel cells are on the fringe of commercialization. The progress made in catalyst and membrane research in the last few years has enabled very high power densities (>1 W/cm2) with moderate efficiencies for the fuel cell (40%). The catalyst loading of electrodes has been reduced to 0.1 mg Pt/cm2 while maintaining a high performance. The price of the perfluorinated sulfonic acid membranes such as the Nafion membrane is expected to decrease, with increasing production, while other proton conducting membrane candidates have been discovered.
Serious problems, however, are encountered when polymer fuel cell technology is scaled up to larger cells and stacks. One of the main problems, in the stacks themselves, is the water management, since the proton conducting membrane must be kept well humidified under operating conditions.
The dominating component, at the internal resistance loss in the stack, is due to the limiting proton conductivity of the membrane. Membranes tend to dry out, especially on the anode side, at high current densities, since proton migration drags water molecules away from the anode.
Drying of the anode does not only affect resistance, but also the kinetics of hydrogen reduction reaction (HRR) at the anode.
Therefore, in attempts to remedy this problem the anode side is often humidified more intensively than the cathode side. The cathode side of the cell can also be pressurized to use the pressure gradient over the membrane to press the water back to the anode. However, it is important that the water management does not impede the gas flow inside the cells.
One solution for this problem would be to use thinner membranes, but this approach has limitations since mechanical rigidity of the membrane must be sufficient.
Another solution is to have a direct water contact with the membrane at the anode side since the water content and conductivity of the membrane are much higher when the membrane is in equilibrium with water. Also, when liquid is evaporated inside the fuel cell, a considerable amount (40-50%) of the heat can be removed from the cell with the produced water vapor.
U.S. Pat. No. 5,958,613 relates to such direct water humidification of fuel cell membranes. Therein, a polymer fuel cell system is disclosed with a capability to moisten the solid-polymer film without providing a special humidifier that humidifies the fuel gas or the oxidizer gas, and that cools down the main cell body channels. In this patent, there is no disclosure of specified operation principles for a fuel cell stack, when the direct humidification is applied.
U.S. Pat. No. 5,935,726 discloses a method and apparatus for improved humidification of membranes in polymer fuel cells by periodically reversing the flow direction of the oxidant stream through a fuel cell flow field. This patent, however, is not concerned with cooling of the fuel cell.